Double shell thickener

ABSTRACT

A double-walled thickener tank has an outer shell bottom and an inner shell bottom spaced to form interconnected or a discrete void space therebetwee. An incompressible load-carrying permeable material such as oil-impregnated sand is compacted in the void space(s) to provide a support for the inner shell bottom and confined fluid in the tank. Any leakage of fluid from the tank through the inner shell bottom is initially confined in the void space and conveyed down a slight slope on the top surface of the outer shell bottom so the leaking fluid drains into an interior space of a double-wall or single-wall discharge cone at the center of the tank where the leak and void spaces are accessed to detect any leaking of fluid from the tank. Various modifications utilize either a replacement of a new inner tank liner, a bottom sheet of imprevious material such as a plastic liner; a series of pie-shaped steel sheets forming each bottom; or a grooved concrete bottom, all providing a flat or inclined void space, with or without load-carrying permeable material therein, which is accessed for leak detection.

This application is a continuation-in-part of U.S. application Ser. No.07/123,745, filed Nov. 23, 1987, now U.S. Pat. No. 4,840,283, issuedJune 20, 1989.

This invention relates to double-wall fluid containment tanks. Moreparticularly, the invention is directed to a double-wall elevated orground-supported thickener, clarifier or other equipment used inliquid-solid separation processes, including waste water treatment.

BACKGROUND OF THE INVENTION

Heretofore, most fluid containment tanks have been comprised of asingle-wall construction including a single-wall side wall and a shellbottom for containing the fluid and for supporting the weight of thefluid in the tank. The shell bottom normally rests on wooden, steel beamor concrete supports resting on pylons or a ground support. When a toxicor other ground water damaging or medically hazardous material is to beconfined, double wall containments have been mandated. Thus in the caseof certain waste water treatment processes the U.S. EnvironmentalProtection Agency (EPA) have required a double-wall containment. Watertreatment equipment as mandated by the EPA typically comprisedouble-wall barriers made up of two totally separate tanks. As anexample, a concrete floor with peripheral vertical concrete walls willbe lined with a metal liner or painted. Inside this large normallyopen-to-the-atmosphere containment, a steel elevated tank will beerected and utilized. Any leakage from the elevated tank will becaptured and monitored by means of the concrete containment.

Other equipment has utilized double containment barriers. Scrubbers usedin the Power industry utilize a liner (a corrosion-resistant thinmaterial) laid directly against a heavier structural load carrying shell(noncorrosion resistant). Leakage is monitored in the annular area. Oiltankers also utilize double-barrier containments.

SUMMARY OF THE INVENTION

A double-wall tank construction is provided in which an inner shellbottom is supported over essentially all of its area and wherein anyleakage from the interior of an overall inner fluid-holding containmentis detectable by centrally accessing the space between the innercontainment and an outer containment. A suitable load-carrying permeablematerial is preferably placed in void spaces formed by spacer barserected between an outer shell bottom and the inner shell bottom or bypermeable material, per se, placed between a ground-supported outerbottom liner and an inner shell, which not only provides overall supportfor the inner shell and its fluid contents, but provides a materialwhich allows for the flow of any fluid leaking from the inner shell intothe space between the shells (or shell and liner) to be detected atmultiple access detection locations or at a single detection location.

In a preferred embodiment the outer shell is first formed on suitableradial support beams which normally are leg-supported. The edges ofpie-shaped steel sheets are mounted on and secured, as by welding, tothe support beams to provide structural and seal integrity of the outershell. Radial and segmented annular support bars are welded to the topof the outer shell and sand or other inert granular material is filledand compacted between the radial and annular support bars which spacethe outer and inner shell bottom from each other. The edges of otherpie-shaped steel sheets which form the inner shell are then secured, asby welding, to the radial support bars to provide structural and sealintegrity for the inner shell. In the case of a liquid-solid thickenersuch as those finding extensive application in waste water treatment theinner shell bottom is formed in a first inverse cone configuration toaccommodate a rotating solids-moving rake structure which transportssettled solids to a central sludge discharge cone attached to andextending through a central aperture in each of the inner shell bottomand outer shell bottom.

Means are provided for accessing one or more locations in the voidspaces between the inner and outer shells for detection of any leaksthrough the inner shell into the void spaces. In the preferredembodiment access is provided by providing a double-wall sludgedischarge cone which allows for drainage of leaked fluid from theinterstices of the permeable material in the void spaces between theinner shall bottom and outer shell bottom to a suitable drainage nozzleextending from an outer wall of the discharge cone. A sludge dischargenozzle extends in the cone through both walls of the discharge cone tofacilitate pumped removal of settled sludge from the discharge coneinterior.

Among the unique features of this invention are: the inner shell isseparated and supported from the outer shell by means of a combinationof moldable incompressible material (such as oil impregnated sand) withor without bearing bars therein; inner tank fluid leakage is collectedin one central area; leakage can freely flow to the central area sincecontinuous annular supports are not required; the tank bottom outershell is normally erected with a combination of flat plates, forexample, as pie-shaped or coned segments or as in the form of a fluidimpervious material such as a plastic liner or grooved a concretebottom. The constructions allow the inner shell to be formed into a coneto enhance the total swept area of a rotating rake arm; and corrosion isminimized by utilizing a minimal number of flow restrictions, i.e.bearing bars, or having the entire inner bottom shell supported on theoil-impregnated sand and on concrete peripheral foundation.

As can be seen, the tankage of the invention may be erected all aboveground or in a formed depression in the ground. An exotic inner tankliner (stainless steel or other material) may be employed with a lessexpensive outer shell thus reducing overall cost. Dual containment mayalso be applied solely to the tank bottoms. The term "thickener" as usedherein means any thickener or clarifier or other double-containmentvessel used in the water, wastewater or chemical or metallurgicalprocess industries which is utilized in liquid-solids separationprocesses.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic half, partial cross-section side view of athickener incorporating the invention.

FIG. 2 is a plan view of an outer shell bottom of a thickener tank at aninterim step of tank erection.

FIG. 3 is a plan view of the tank inner shell bottom in tank assembledcondition.

FIG. 4 is a cross-section view of a radial section of the tank bottomtaken on the lines 4--4 of FIG. 1.

FIG. 5 is a cross-section view of an arcuate section of the tank bottomtaken on the lines 5--5 of FIG. 1.

FIG. 6 is a cross-section side partial view of the discharge coneshowing its connection with the tank outer and inner shell bottoms.

FIG. 7 is a cross-section view of a chord section taken on the lines7--7 of FIG. 2 showing a series of spacer bars between the shells.

FIG. 7A is a cross-section view of a chord section similar to FIG. 7with sand filling the void spaces between the outer and inner shell.

FIG. 8 is a cross-section view of an arcuate section taken on the lines8--8 of FIG. 2 showing a series of spacer bars between the shells.

FIG. 8A is a cross-section view of an arcuate section similar to FIG. 8with sand filling the void spaces between the outer and inner shells.

FIG. 9 is a schematic cross-section side view along a radial showing amodified scalloped form of outer liner.

FIG. 10 is a schematic cross-section side view of an embodiment whereina replacement of a new inner tank liner is employed to effect aconversion of a single bottom tank to dual containment tankage.

FIG. 11 is a schematic cross-section side view of a center column-typethickener employing the invention.

FIG. 12 is a schematic cross-section side view of an embodiment of theinvention employing a sheet of fluid impenetrable material as the outertank bottom.

FIG. 13 is a schematic cross-section side view of a further embodimentof the invention utilizing a grooved concrete outer liner bottom.

FIG. 14 is a schematic plan view of the grooved surface of the outerbottom of the FIG. 13 liner.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As shown in FIG. 1 a double-wall bottomed thickener 10 comprises acircular vertical tank wall 11 surrounding and attached by welding orthe like to the peripheral edges of an outer shell bottom 12 and aninner shell bottom 14 spaced therefrom by arcuate, i.e. preferablystraight, sections extending laterally across an arc segment, spacers 40and radial spacers 50. Each of the shown outer long arcuate spacers 40and inner short arcuate spacers 40a extend for a distance less than thedistance between adjacent radial spacers 50 forming gaps 70 therebetween allowing flow of leaking fluid therepast. The inner shell bottomhas an inverse conical top surface 19 which receives settled solids orsludge from a solids-containing fluid contained in the tank formed bywall 11 and inner shell bottom 14. Wall 11 may be a double-wall, theinterior 6 of which between wall 11 and wall 9 is in flow communicationwith the space between outer shell bottom 12 and inner shell bottom 14.

As well know in the thickener art, typically a walkway truss 7 bridgesacross the tank and supports a rake drive mechanism 16. A centralturbine shaft 15 extends into the tank from the drive mechanism androtates one or more rake arms 17 affixed to the shaft end. Rake blades18 extend downwardly along the rake arm. Blades 18 may include stainlesssteel or rubber squeegees on their bottom edges. Blade 18 rotates aroundthe tank bottom surface 19 to transport settled solids radially inwardlyinto a central discharge cone 20. Solid line arrows 21 illustrate themovement of sludge from surface 19 to the interior 22 of the dischargecone. A sludge outlet 23 extends from the cone interior and is connectedto a sludge pump (not shown) for removal of settled sludge from thethickener and the discharge cone. Not shown in FIG. 1 are conventionallyemployed peripheral tank launders, weirs, sampling ports, reactionwells, baffling and influent piping, for example.

In order to support the weight of the fluid in the thickener 10 in thepreferred embodiments, an inert, non-compressable, load-carryingmaterial 60 fills the void spaces 36 between the shell bottoms 12, 14and between the spaced arcuate and radial spacers extending in the spacebetween the shell bottoms. In the preferred embodiment the void spacesare filled and compacted with oil-impregnated sand. Clean 30- meshsiliceous sand saturated with a heavy-base petroleum oil (motor oil) maybe used.

Any leakage of fluid in the thickener tank either through the inner wallor the inner shell bottom 14 will drain into the permeable sand 60 andbe conveyed by the inward slope, typically being at a slope rate ofabout 1:12 to about 2.5:12, along the outer shell bottom through thesand in void spaces 36 and past the arcuate and radial spacers 40, 50into a downwardly sloping space 37 between the double walls of thedischarge cone, and then to a leak detection fluid outlet nozzle 35which accesses the void spaces between the double side walls, thedouble-shell bottoms and the double discharge cone walls.

While the tank has been described in terms of a thickener tank, the tankmay be used for clarification, floatation or as a simple storage tankfor liquids or liquid - solids slurries.

Fabrication of the aforesaid structure is performed by providingsuitable concrete pedestals 32, 33 on a concrete or other suitableground support surface 46. Outer support legs 31, spaced around the tankperiphery, support one end of a series of radial support beams 30 whichare secured, as by welding, at their opposite ends onto the outer shell24 of discharge cone 20. Discharge cone 20 may be supported by a centersupport leg 34 extending above pedestal 33.

As shown in FIG. 2 a series of pie-shaped steel sheets 12a, 12b, . . . ,12x, then are placed on and connected, as by radial edge welding, to theradial support beams 30 forming the outer shell bottom 12. The innerperipheral edges of the sheets are fitted to and connected, as bywelding, to an angular flange extension 41 of the cone shell 24. Radialspacer bars 50 and segments of arcuate spacer bars 40 are spacedlypositioned on the top of the outer shell bottom and sand 60 impregnatedwith oil for corrosion resistance is placed and compacted in the voidspaces between and even with the top level of the upstanding spacers 40,50. In the preferred embodiment the sand is smoothed into an inversecone-like top surface corresponding to the desired slope of the topsurface 19 of inner shell bottom 14. Sand is only used on the flattersurfaces of the tank bottom, not the central cone area. As seen in FIG.7A the sand is brought up to a level adjacent to the tops of the barsand the bars remain exposed for welding, FIG. 7 shows bars of variousheights between which sand is placed and contoured as seen in FIG. 7A.

Pie-shaped steel sheets 14a, 14b, . . . , 14x are then positioned withtheir edges abutting and in engagement with the tops of the radialspacer bars 50. The inner peripheral edges of the sheets 14a to 14x areinserted under an angular annular flange 39 of inner cone shell 25 (FIG.6). The steel sheets are then secured in place by a seam weld 13 (FIG.3) preferably connecting the adjacent abutting steel sheets to theirunderlying radial spacer bar 50. The inner shell bottom is spaced abovethe outer shell bottom a distance of 1/4"-1" depending upon thediametric size of the tank and where along the radial surface of thetank bottom surface 19 the measurement is taken. In a typical 25 foot IDtank having a 10 foot vertical wall, 1/4" thick steel sheets areutilized for the inner and outer shell bottoms.

The walls of the tank are also of double-wall construction having gappedvertical spacer bars 6 therebetween for providing the void between theliner and outer shell. However, in contrast to the bottom this voidtypically is not filled with sand or other material. In the event thatadditional wall strength is required, it is however contemplated that anincompressible yet permeable material such as sand or a perforatedhoneycomb structural insert could be provided in this void in the wall.

One or more spaced vacuum release fittings 8 extend from the tankexterior into the space between walls 9 and 11 to provide for relief ofany vacuum in the void spaces and provide air column pressure to assistin leak drainage to the discharge cone.

The use of double-wall construction with sand (or other suitableload-carrying, permeable material) therebetween enables the uppersurface 19 of the bottom of the tank to assume the desired shape. Asbest shown in FIG. 5 and because the radial support beams for theelevated tank and the pie-shaped sheet steel outer shell segments are ofgenerally planar construction, their assembly to form the bottom of thetank cannot conform to a conical surface. The use of a flowable, yetload-carrying material such as sand between the inner liner and outershell allows the necessary dimensional accommodation between the desiredupper conical surface and lower planar approximation thereto. Thisexplains the range of distances between corresponding points on theupper liner and lower outer shell. Of course, a conical surface asopposed to a planar approximation thereof enables the rake arms to sweepthe bottom 19 of the tank clean rather than leaving pockets of unremovedmaterial which can then ferment. This is important for tanks used tohandle organic matter which could become septic if left unremoved.

The double-wall construction allows any leakage through the liner to becollected and detected at an exit from the space between the inner shellbottom and the outer shell bottom to prevent contamination of theenvironment with hazardous chemicals or other materials as may berequired by the EPA.

FIG. 4 shows a detail of the spacing between outer shell bottom 12 andinner shell bottom 14 by arcuate spacer bars 40. Oil-impregnated sand 19is shown compacted in the void space between bars 40. FIG. 5 shows adetail of the above spacing adjacent a radial spacer bar 50 withoil-impregnated sand 60 in the adjacent void spaces.

FIG. 6 illustrates in detail the connection of the inner annularperipheries of the shell bottoms with the double-walled discharge cone20. The discharge cone has circular flanges 39, 41 which receives theshell bottoms. A series of 1/8"×2"×2" spaced spacer bars 26 allowdrainage of leaked fluid from the space 36 between the shell bottoms 12,14 into the space 37 between cone walls 24, 25. As shown by the dottedarrows 29 any leaks from the tank 10 through inner shell bottom 14 (orthrough vertical inner wall 9 into space 36--FIG. 1) is drained throughthe granulated material and conveyed by the slope of the inner surfaceof cone wall 24 to the bottom of the cone where the leakage and spaces36, 37 are accessed by nozzle 35. A suitable fixed screen 28 retains anysand which is purposely or inadvertently found in the cone interior wallspace 37. A suitable standard liquid detector and monitor with suitablealarm, attached to nozzle 35 or an enclosed Plexiglas sight glass may beprovided to visually observe any leakage.

The double-wall construction allows a supply of air or nitrogen or othersuitable gas to be provided to the void space 36 through inlet 38 orthrough one of the valve fittings 8 for detecting the existence of leaksin the inner liner by visually observing gas bubbles appearing at thetop of the liquid in the tank or by a drop in supply gas pressureindicative of a leak in the fluid containment.

The use of spacer bars of a predetermined length, configuration, andplacement in the void spaces can enable the tank to be divided into anumber of independent void spaces for detecting leaks in predeterminedportions of the tank. In this regard, the spacer bars may be used todivide the bottom of the tank into relatively small pie-shaped sectionsand the wall divided into a plurality of arcuate nonconnected sectionsif so desired. As seen by dash lines 37a in FIG. 3, similar discretesections can be provided (FIG. 6) in the discharge cone by providinglongitudinal spacers 47 in the cone space 37 connected to each radialbar 50, and separate leak detector nozzles attached to each discretepie-shaped segment section, formed between each of the longitudinal conespacers, at the cone discharge bottom. Referring to the drawing and thetank liner detail thereof, it is apparent that the wall voids can beseparated and made independent of the void spacers in the bottom of thetank. Alternatively the wall void spaces can be placed in communicationwith a suitable nozzle(s) accessing the void space(s).

As seen in FIG. 9, if a scalloped bottom is used (as is done frequentlyon elevated tanks greater than 60 foot diameter) the inner sloped liner19 may be spaced from 1/4" to several feet (adjacent the low point ofthe scalloped section 12a) from the bottom tank shell.

FIG. 10 illustrates the use of the invention to effect a conversion of asingle tank bottom to dual bottom containment tankage. The original tankcontains a cylindrical sidewall 50, a tank bottom 51, a sludge dischargecone 52 and a sludge exit pipe 53, all typically mounted on individualconcrete or other piers 54 or a continuous ring support structure.

After repair of any leak in bottom 51, a vertical or sloped collar 56 isaffixed by welding, or the like, to cone 52. A layer of compactedload-carrying permeable material 55, such as oil-impregnated sand, isthen placed over bottom 51 and around the collar 56. A new replacementinner bottom 57 normally made of steel sheets is positioned to rest onand be supported by the compacted sand and welded to the top of collar56 and to the tank sidewall. This forms a new fluid-tight tank bottom57. Lastly, a leak detection pipe 58 is provided extending through theoriginal tank sidewall 50 accessing the compacted sand void volumebetween the tank bottoms 57 and 51 so as to detect any leaks through thenew inner bottom 57. While shown as a flat surface, the bottom 57 may beinclined as is the inner bottom in FIG. 1 and may include a rakestructure, also as shown in FIG. 1.

FIG. 11 shows a center column-type thickener/clarifier including a steelcylindrical vertical tank wall 60, a steel inner liner bottom 61, asteel outer liner bottom 62, compacted oil-impregnated sand 63 or otherpermeable material in the void space between the bottoms, a double-wallcone sludge pit 64 offset from an influent column 65 which is fedinfluent by a bottom inlet pipe 66. A leak detection pipe 67concentrically surrounds pipe 66 and the annulus therebetween accessesthe void space 68 between the bottoms 61 and 62. A sludge exit pipe 69connects the interior of cone sludge pit 64 to discharge while anotherconcentric pipe 70 accesses the void space between the walls of the conesludge pit and also the void space between the bottoms. Suitabledetection means (not shown) detects leaks directed outwardly in theannuli between pipes 66, 67 and pipes 69, 70. A concrete center pier 71and peripheral pier(s) or ring wall 72 support the tank, influent columnand sludge pit. The tank wall is shimmed to the ring wall at 73 andgrout 74 applied to seal the peripheral edge between the bottoms.

The FIG. 12 embodiment provides double-containment by utilizing a sheetof fluid impervious material such as a polyethylene or other plasticliner 80 typically of about 40 mils thickness which is placed over andis supported by a bed 81 of compacted oil-impregnated sand or otherpermeable material properly contoured to approximate the desired finalslope of the inner bottom of the tank. A radial ditch is provided toreceive a sludge exit pipe 82. A concentric leak detection pipe 83accesses a compacted sand-containing void volume above the plastic liner80 which is formed in the space above the liner and an inner steel tankbottom 84 containing a single wall sludge discharge cone 85. Compactedsand 86 supports bottom 84 and allows any leaks through bottom 84 to bedirected to the annulus between pipes 82, 83 for detection. A portion ofliner 80 is suitably clamped and sealed by a ring clamp 87 or the liketo the exterior of pipe 83 and the outer periphery of the liner sealedunder the outer periphery of bottom 84 and cylindrical wall 88 on thetop of a ring wall 89 supported by the ground 90.

FIG. 13 shows a further embodiment in which a circular concrete base 91is poured at ground level 92. A series of radial spaced grooves 93 areprovided on the top surface of the base 91 which intersect with sidegrooves 94 (FIG. 14). An inner steel shell bottom 95 is positioned overand is supported by the concrete base 91 either directly or on a bed ofcompacted sand therebetween. A cylindrical tank wall 96 and a singlewall sludge cone 97 completes the inner tank. A sludge exit pipe 98extends outwardly from the inner surface of the sludge cone. A leakdetection pipe 99 extends from a sump 75 under the sludge conecomprising a grooved central surface of the base 91 which is in flowcommunication with grooves 93, 94 so as to convey any fluids from thetank which may have leaked through bottom 95 to a position exterior ofthe tank for detection.

The above description of embodiments of this invention is intended to beillustrative and not limiting. Other embodiments of this invention willbe obvious to those skilled in the art in view of the above disclosure.

We claim:
 1. A dual-containment tank system for a solids/liquid loaded thickener/clarifier wherein solids settle within said tank system comprising:means for ground supporting a cylindrical tank shell having an upstanding continuous side wall; an inner tank shell bottom sealingly extending across a bottom of said continuous side wall; a sludge drain extending through said shell bottom for removal of settled solids; an outer tank shell bottom generally co-extensive with and spaced from said first inner tank shell bottom to form a void space between said shell bottoms; the shape of the upper surface of the outer tank shell bottom corresponding generally to the shape of the lower surface of the inner tank shell bottom, with the inner tank shell bottom being generally nested in the outer tank shell bottom; means accessing said void space for detecting leaks of liquid confined by said continuous side wall and said inner tank shell bottom, further comprising a bed of compacted permeable material in said void space for providing support of said inner tank shell bottom; and in which said sludge drain comprises a spaced double-walled cone extending from said inner tank shell bottom, an interior of said cone being in flow communication with said void space and said means for detecting leaks.
 2. A dual-containment tank system for a solids/liquid loaded thickener/clarifier wherein solids settle within said tank system comprising:means for ground supporting a cylindrical tank shell having an upstanding continuous side wall; an inner tank shell bottom sealingly extending across a bottom of said continuous side wall; a sludge drain extending through said shell bottom for removal of settled solids; an outer tank shell bottom generally co-extensive with and spaced from said first inner tank shell bottom to form a void space between said shell bottoms; the shape of the upper surface of the outer tank shell bottom corresponding generally to the shape of the lower surface of the inner tank shell bottom, with the inner tank shell bottom being generally nested in the outer tank shell bottom; means accessing said void space for detecting leaks of liquid confined by said continuous side wall and said inner tank shell bottom; and in which said sludge drain comprising a spaced double-wall cone and a generally cylindrical discharge tube extending from an inner surface of said cone to sludge discharge exterior of said tank and in which said means accessing said void space includes a generally concentrically spaced leak detection tube surrounding said cylindrical discharge tube and extending to a position exterior of said cylindrical tank shell.
 3. The tank system of claim 2 further comprising a bed of compacted permeable material in said void space and surrounding said discharge tube and leak detection tube.
 4. A dual-containment tank system for a solids/liquid loaded thickener/clarifier wherein solids settle within said tank system comprising:means for ground supporting a cylindrical tank shell having an upstanding continuous side wall; an inner tank shell bottom sealingly extending across a bottom of said continuous side wall; a sludge drain extending through said shell bottom for removal of settled solids; an outer tank shell bottom generally co-extensive with and spaced from said first inner tank shell bottom to form a void space between said shell bottoms; the shape of the upper surface of the outer tank shell bottom corresponding generally to the shape of the lower surface of the inner tank shell bottom, with the inner tank shell bottom being generally nested in the outer tank shell bottom; means accessing said void space for detecting leaks of liquid confined by said continuous side wall and said inner tank shell bottom; and in which said sludge drain comprises a double-wall cone having an inner cone and a spaced outer cone; a sludge discharge tube extending through said outer cone; and in which said means for accessing extends into a second void space between said cones, said second void space being in flow communication with said shell bottoms void space.
 5. A dual-containment tank system for a solids/liquid loaded thickener/clarifier wherein solids settle within said tank system comprising:means for ground supporting a cylindrical tank shell having an upstanding continuous side wall; an inner tank shell bottom sealingly extending across a bottom of said continuous side wall; a sludge drain extending through said shell bottom for removal of settled solids; an outer tank shell bottom generally co-extensive with and spaced from said first inner tank shell bottom to form a void space between said shell bottoms; the shape of the upper surface of the outer tank shell bottom corresponding generally to the shape of the lower surface of the inner tank shell bottom, with the inner tank shell bottom being generally nested in the outer tank shell bottom; means accessing said void space for detecting leaks of liquid confined by said continuous side wall and said inner tank shell bottom; and in which said means for supporting comprises an inverse conical ground bed and a concrete support ring surrounding said bed and positioned under said continuous side wall; wherein said outer tank shell bottom comprises a sheet of fluid impermeable material laid on said ground bed; and wherein said void space is filled with a permeable material between said sheet material and an underside of said inner tank shell bottom.
 6. The tank system of claim 5 in which the sheet of fluid impermeable material is a plastic liner.
 7. A dual-containment tank system for a solids/liquid loaded thickener/clarifier wherein solids settle within said tank system comprising:means for ground supporting a cylindrical tank shell having an upstanding continuous side wall; an inner tank shell bottom sealingly extending across a bottom of said continuous side wall; a sludge drain extending through said shell bottom for removal of settled solids; an outer tank shell bottom generally co-extensive with and spaced from said first inner tank shell bottom to form a void space between said shell bottoms; the shape of the upper surface of the outer tank shell bottom corresponding generally to the shape of the lower surface of the inner tank shell bottom, with the inner tank shell bottom being generally nested in the outer tank shell bottom; means accessing said void space for detecting leaks of liquid confined by said continuous side wall and said inner tank shell bottom; and further including an influent column vertically extending up within said tank and through said inner tank shell bottom and said outer tank shell bottom; said sludge drain peripherally surrounding said influent column; and an influent pipe extending from a position below said shell bottoms to a lower portion of said influent column, said influent pipe being surrounded by a concentric leak detector pipe forming a portion of said means accessing said void space.
 8. The tank system of claim 7 wherein said means for ground supporting includes a central pier supporting said influent column, said influent pipe, said leak detector pipe and an inner periphery of said shell bottoms; said sludge drain extending through said central pier.
 9. A dual containment tank system for a solids/liquid loaded thickener/clarifier comprising:means for ground supporting a cylindrical tank having an upstanding continuous side wall; a first tank bottom extending across a bottom of said continuous side wall; a bed of material which is permeable to a flow of liquid therethrough extending across and above said tank bottom; a second tank bottom sealingly extending across said continuous side wall over said permeable bed; a sludge drain extending through said second tank bottom for removal of solids settling out on an upper surface of said second tank bottom from the tank; means accessing said bed for detecting any leaks through said second tank bottom into said bed; further comprising means for supplying nitrogen gas into said permeable bed; and wherein said sludge drain includes a double wall cone having a gas inlet opening in communication with a space between said cone walls and with said means supplying nitrogen being in communication with said gas inlet opening.
 10. A dual-containment tank system for a solids/liquid loaded thickener/clarifier comprising:means for ground supporting a cylindrical tank having an upstanding continuous side wall; a first tank bottom extending across a bottom of said continuous side wall, said first tank bottom being formed of concrete; a second tank bottom sealingly extending across said continuous side wall forming said tank and confining a solids/liquid, said second tank bottom extending above and forming a space between said bottoms and being supported by said first tank concrete bottom; a sludge drain extending through said second tank bottom for removal of solids settling out on an upper surface of said second tank bottom; means in communication with said space for detecting a leak of liquid from said solids/liquid through said second tank bottom into said space; and wherein said first tank concrete bottom includes spaced grooves on a top surface thereof forming said space.
 11. The tank system of claim 10 wherein said spaced grooves extend from adjacent the periphery of said first tank concrete bottom to said sludge drain and being in flow communication with said means accessing said space. 